Semiconductor lasers are important components of optical communication systems. They serve, for example, as sources of light to be digitally modulated and transmitted along an optical fiber network. In wavelength division multiplexed optical systems several different closely-spaced wavelength signals are transmitted at the same time. Semiconductor lasers provide the different wavelengths of light for carrying these signals.
Semiconductor distributed feedback lasers (DFB lasers) are particularly useful in optical communications because they have very narrow linewidths which permit the transmission of numerous different wavelength channels without substantial overlap. A DFB laser typically includes a periodic grating sufficiently close to its active region to interact with the lasing optical field. An active region comprising a plurality of quantum wells is typically grown over the grating. The grating narrows the lasing linewidth so that many different wavelength channels can be transmitted for optical fiber communication As a consequence, there is a demand for DFB semiconductor lasers in a variety of different, closely-spaced wavelengths.
One problem in providing needed lasers is the difficulty of fabricating the lasers in different wavelengths. The current practice is to make a batch of many single wavelength lasers on a single wafer. Gratings of predetermined pitch targeted for a particular wavelength are holographically formed over the entire wafer. But to supply lasers with different wavelengths--which may be separated by as much as 40 nanometers--one must fabricate many wafers and a very large number of lasers. Moreover the demands for lasers for different wavelengths are not equal. Therefore by the current practice, one is left with a large inventory of lasers (wavelengths not in demand) and of wafers which, due to slight inaccuracies, cannot meet very demanding requirements of correct wavelength.
Efforts have been made to circumvent this problem by using E-beam lithography to write different grating pitches on a single wafer. The technique, however, involves high capital cost, low throughput and high cost of maintenance. Accordingly there is a need for a new method of making multiple wavelength lasers on a single wafer.